Enhancing robustness of soi substrate containing a buried n+ silicon layer for cmos processing

ABSTRACT

A silicon buffer layer selected from undoped silicon, p-doped silicon or a multilayered stack of, in any order, undoped silicon and p-doped silicon is provided between an n +  silicon layer and an oxide layer of an SOI substrate. The presence of the silicon buffer layer reduces electron injection into the oxide layer during device processing which requires an electric field.

BACKGROUND

The present application relates to semiconductor technology, and moreparticularly to a semiconductor-on-insulator substrate (SOI) containingan undoped and/or p-doped silicon buffer layer located between an n⁺silicon layer and an oxide layer of the SOI substrate. The presentapplication also relates to a method of forming such a SOI substrate.The present application also provides an annealing process that can beused to minimize or eliminate electron traps in the oxide layer of anySOI substrate.

Semiconductor-on-insulator (SOI) technology refers to the use of alayered semiconductor-insulator-semiconductor substrate such as forexample, silicon-oxide-silicon, in place of a conventional bulksemiconductor (i.e., Si) substrate in semiconductor manufacturing,especially microelectronics. The implementation of SOI technology is oneof several manufacturing strategies employed in the semiconductorindustry to allow for the continued miniaturization of microelectronicdevices. Reported benefits of SOI technology relative to conventionalbulk complementary metal oxide semiconductor (CMOS) processing include,for example, lower parasitic capacitance due to isolation from the bulksemiconductor substrate, which improves power consumption at matchedperformance, and/or resistance to latch-up due to complete isolation ofn- and p-well structures, and/or higher performance at equivalent VDD,and/or reduced temperature due to no doping, and/or better yield to highdensity, and/or lower leakage currents due to isolation thus higherpower efficiency, and/or inherent radiation hardened.

From a manufacturing perspective, SOI substrates are compatible withmost conventional fabrication processes. In general, an SOI-basedprocess may be implemented without special equipment or significantretooling of an existing factory.

Despite the above, improvements with existing SOI substrates are neededparticularly concerning the minimization or elimination of electrontraps in the buried oxide layer of SOI substrates. Also needed is a newand improved SOI substrate that has enhanced robustness and improvedburied oxide degradation.

SUMMARY

A silicon buffer layer selected from undoped silicon, p-doped silicon ora multilayered stack of, in any order, undoped silicon and p-dopedsilicon is provided between an n⁺ silicon layer and an oxide layer of asemiconductor-on-insulator (SOI) substrate. The presence of the siliconbuffer layer reduces electron injection into the oxide layer duringdevice processing which requires an electric field.

In one aspect of the present application, a semiconductor-on-insulator(SOI) substrate that has enhanced robustness and improved buried oxidedegradation is provided. In one embodiment of the present application,the SOI substrate includes from bottom to top, a silicon handlesubstrate, an n⁺ silicon layer, a silicon buffer layer, an oxide layer,and a p⁻ silicon layer. The silicon buffer layer is selected fromundoped silicon, p-doped silicon or a multilayered stack of, in anyorder, undoped silicon and p-doped silicon.

In another aspect of the present application, a method of forming asemiconductor-on-insulator (SOI) substrate is provided. In oneembodiment of the present application, the method includes providing afirst wafer comprising an oxide layer and a precursor p⁻ silicon layerand a second wafer comprising, from bottom to top, a silicon handlesubstrate, an n⁺ silicon layer and a silicon buffer layer, the siliconbuffer layer is selected undoped silicon, p-doped silicon or amultilayered stack of, in any order, undoped silicon and p-dopedsilicon. Next, a surface of the oxide layer of the first wafer isbrought into intimate contact with a surface of the silicon buffer layerof the second wafer. The first wafer is then bonded to the second waferand a portion of the precursor p⁻ silicon layer is removed to provide asemiconductor-on-insulator substrate comprising from bottom to top, thesilicon handle substrate, the n⁺ silicon layer, the silicon bufferlayer, the oxide layer, and a remaining portion of the precursor p⁻silicon layer.

In yet another aspect of the present application, a method is providedthat minimizes or eliminates electron traps in an oxide layer of an SOIsubstrate containing such traps. In one embodiment of the presentapplication, the method includes providing a semiconductor-on-insulator(SOI) substrate having an oxide layer that contains a first quantity ofelectron traps. The method continues by annealing the SOI substrate toprovide the oxide layer with a second quantity of electron traps,wherein the second quantity of the electron traps is less than the firstquantity of the electron traps.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a cross sectional view of an exemplarysemiconductor-on-insulator (SOI) substrate including, from bottom totop, a p⁻ silicon handle substrate, an oxide layer, and a p⁻ siliconlayer.

FIG. 2 is a cross sectional view of another exemplarysemiconductor-on-insulator (SOI) substrate including, from bottom totop, a p⁻ silicon handle substrate, an n⁺ silicon layer, an oxide layer,and a p⁻ silicon layer.

FIG. 3 is a cross sectional view of an exemplarysemiconductor-on-insulator (SOI) substrate of including, from bottom totop, a silicon handle substrate, an n⁺ silicon layer, a silicon bufferlayer, an oxide layer, and a p⁻ silicon layer in accordance with anembodiment of the present application.

FIG. 4A is a cross sectional view of a first wafer that can be used inproviding the exemplary semiconductor-on-insulator substrate illustratedin FIG. 3.

FIG. 4B is a cross sectional view of a second wafer that can be used inproviding the exemplary semiconductor-on-insulator substrate illustratedin FIG. 3.

FIG. 5 is a cross sectional view of the first and second wafers shown inFIGS. 4A and 4B after bringing the two wafers into intimate contact witheach other, bonding the first and second wafers together, and removing aportion of a precursor p⁻ silicon layer of the first wafer.

DETAILED DESCRIPTION

The present application will now be described in greater detail byreferring to the following discussion and drawings that accompany thepresent application. It is noted that the drawings of the presentapplication are provided for illustrative purposes only and, as such,the drawings are not drawn to scale. It is also noted that like andcorresponding elements are referred to by like reference numerals.

In the following description, numerous specific details are set forth,such as particular structures, components, materials, dimensions,processing steps and techniques, in order to provide an understanding ofthe various embodiments of the present application. However, it will beappreciated by one of ordinary skill in the art that the variousembodiments of the present application may be practiced without thesespecific details. In other instances, well-known structures orprocessing steps have not been described in detail in order to avoidobscuring the present application.

It will be understood that when an element as a layer, region orsubstrate is referred to as being “on” or “over” another element, it canbe directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “beneath” or “under” another element, it can bedirectly beneath or under the other element, or intervening elements maybe present. In contrast, when an element is referred to as being“directly beneath” or “directly under” another element, there are nointervening elements present.

As shown in FIG. 1, SOI substrates conventionally use a p⁻ siliconhandle substrate 10 under an oxide layer 12 and a p⁻ silicon layer 14above the oxide layer 12; oxide layer 12 may also be referred to hereinas a buried oxide (BOX) layer. The SOI substrate shown in FIG. 1 can beformed by wafer bonding or by SIMOX (Separation by IMplantation ofOXygen). SOI material technology has been optimized since its inceptionto provide high yield SOI based CMOS products. However, a structuralchange as illustrated in FIG. 2 was introduced into the SOI structureshown in FIG. 1 for 22 nm and beyond CMOS technology. Notably, theaforementioned structure change, which is illustrated in FIG. 2,included the introduction of a several micron thick n⁺ silicon layer 16between the BOX layer 12 and the p silicon handle substrate 10; the SOIsubstrate shown in FIG. 2 can be formed by wafer bonding. Thisstructural change introduced a profound change in electric field acrossthe BOX layer 12 and provided a pathway for electrons to be injectedinto the BOX layer 12 from the p⁻ silicon handle substrate 10 and holesto be injected from the overlaying p⁻ silicon layer 14 under nominalplasma processes (grounded substrate). In the presence of traps in theBOX layer 12, accelerated degradation and failure of the BOX layer 12typically occurs. Such failure can be determined by time dependentdielectric breakdown (TDDB) measurements.

The present application provides an annealing process that can be usedto improve existing SOI substrates such as shown in FIG. 2 by minimizingor eliminating electron traps in the buried oxide layer of such SOIsubstrates as well as providing a new and improved SOI substrate thathas enhanced robustness and improved buried oxide degradation ascompared to existing SOI substrates. Notably, the present applicationprovides three embodiments that can be used to control premature BOXdegradation. In one embodiment (I), the introduction of a thin siliconbuffer layer, as described below, can be provided above the n⁺ siliconlayer of a SOI substrate such that electron injection from the n⁺silicon layer into the BOX layer is minimized. In a second embodiment(II), a high temperature annealing of an SOI substrate can be used tominimize or eliminate electron traps in the BOX layer without making anychanges to the n⁺silicon layer and/or the silicon handle substrate.Lastly, and in a third embodiment (III), a combination of embodiments(I) and (II) can be used.

Applicant has shown that I-V characteristics of the virgin oxide used toform BOX layers show ideal Fowler-Nordheim breakdown behavior withoutany noticeable trap related activity as evident from I-V vs. temperaturemeasurements. However, I-V characteristics of the same oxide layer in anSOI configuration show increased leakage as a function of measurementtemperature indicating degradation of the oxide due to H⁺ implantationused to form the SOI substrate and apparent inadequate post-implantannealing. Another important aspect of the present application isrelated to the control of the bonding bottom BOX/n⁺ silicon or handlesubstrate interface. Since BOX degradation is related to chargeinjection, lack of control of this interface may lead to variability incharge injection and thus spatially variable BOX degradation and/or BOXfailure.

Reference is first made to FIG. 3, which illustrates an exemplarysemiconductor-on-insulator (SOI) substrate of the present application.The exemplary semiconductor-on-insulator (SOI) substrate illustrated inFIG. 3 includes from bottom to top, a silicon handle substrate 30, an n⁺silicon layer 32, a silicon buffer layer 34, an oxide layer 36, and a p⁻silicon layer 38. In accordance with this embodiment of the presentapplication, the presence of the silicon buffer layer 34 aids incontrolling the premature degradation of the oxide layer 36. In such anembodiment, the silicon buffer layer 34 minimizes injections ofelectrons from the n⁺ silicon layer 32 into the oxide layer 36.

The various silicon layers (i.e., layers 10, 14, 16, 30, 32, 34 and 38)that comprise the SOI substrates shown in FIGS. 1, 2, and 3 may comprisea same or different crystal orientation. For example, the crystalorientation of the various silicon layers (i.e., layers 10, 14, 16, 30,32, 34 and 38) that comprise the SOI substrates shown in FIGS. 1, 2, and3 may be {100}, {110}, or {111}. Other crystallographic orientationsbesides those specifically mentioned can also be used in the presentapplication. Typically, silicon layers 10 and 16 or silicon layers 30,32 and 34 comprise a same crystal orientation.

The various silicon layers (i.e., layers 10, 14, 16, 30, 32, 34 and 38)of the SOI substrates shown in FIGS. 1, 2 and 3 may be singlecrystalline silicon, polycrystalline silicon, or amorphous silicon.Typically, at least the topmost silicon layer (i.e., layers 16 and 38)of the SOI substrates shown in FIGS. 1, 2 and 3 is a single crystallinesilicon.

The term “n” or “n-type” when used in describing a silicon layer of theSOI substrates shown in FIGS. 1, 2 and 3 denotes the addition ofimpurities that contribute free electrons to an intrinsic semiconductor.For silicon, n-type dopants include at least one element from Group VAof the Periodic Table of Elements (e.g., P and/or As). The term “p” or“p-type” refers to the addition of impurities to an intrinsicsemiconductor that creates deficiencies of valence electrons. Forsilicon, p-type dopants include at least one element from Group IIIA ofthe Periodic Table of Elements (e.g., Al and/or B).

The oxide layer (i.e., layer 12 or 36) of the SOI substrates shown inFIGS. 1, 2 and 3 may be a crystalline or non-crystalline oxide; oxidelayers 12 and 36 may be referred to herein as BOX layers since the oxidelayer is located between silicon layers. In one embodiment, the oxidelayer (i.e., layer 12 or 36) of the SOI substrates can be composed ofsilicon dioxide. In some embodiments (not shown), a nitride layer forexample, silicon nitride or boron nitride, can be formed atop the oxidelayer (i.e., layer 12 or 36) and positioned between the oxide layer andthe topmost silicon layer (i.e., layer 14 or 38) of the SOI substrate.

Throughout the present application, the term p⁻ silicon denotes asilicon layer that has a p-type dopant present in a concentration from10¹⁴ atoms/cm³ to 10¹⁹ atoms/cm³. In one example, the p-type dopantconcentration for the p⁻ silicon layers in the SOI substrates shown inFIGS. 1, 2 and 3 can be from 10¹⁴ atoms/cm³ to 10¹⁸ atoms/cm³.

Throughout the present application, the term n⁻ silicon layer denotes asilicon layer that has an n-type dopant present in a concentration from10¹⁴ atoms/cm³ to 10¹⁹ atoms/cm³. In one example, the n-type dopantconcentration for the n⁻ silicon layers in the SOI substrates can befrom 10¹⁴ atoms/cm³ to 10¹⁸ atoms/cm³.

Throughout the present application, the term n⁺ silicon denotes asilicon layer that has a n-type dopant present in a concentration from10¹⁴ atoms/cm³ to 10²⁰ atoms/cm³. In one example, the n-type dopantconcentration for n⁺ silicon layers in the SOI substrates shown in FIGS.1, 2 and 3 can be from 10¹⁸ atoms/cm³ to 10²⁰ atoms/cm³. The n⁺ siliconlayer has a higher n-type dopant concentration than the n⁻ siliconlayer.

Referring back to FIG. 3 each of the silicon handle substrate 30, the n⁺silicon layer 32, the silicon buffer layer 34, and the p⁻ silicon layer38 may be comprised of single crystalline silicon. In some embodimentsof the present application, the silicon handle substrate 30, the n⁺silicon layer 32, and the silicon buffer layer 34 have an epitaxialrelationship, same crystallographic orientation, with each other.

In one embodiment of the present application, the silicon handlesubstrate 30 is undoped. In another embodiment of the presentapplication, the silicon handle substrate 30 is a p silicon handlesubstrate. In yet another embodiment, the silicon handle substrate 30 isan n⁻ silicon handle substrate.

In one embodiment of the present application, the silicon buffer layer34 consists of undoped Si. In another embodiment, the silicon bufferlayer 34 consists of p-doped silicon having a p-dopant concentration offrom 10¹⁴ atoms/cm³ to 10¹⁸ atoms/cm³. Other p-type dopantconcentrations that are lesser than, or greater than, the aforementionedrange can be used for the p-type dopant concentration of silicon bufferlayer 34. In some embodiments, the silicon buffer layer 34 includes amaterial stack of, in any order, undoped silicon and p-doped silicon.The number of layers of undoped silicon and p-doped silicon within sucha multilayered silicon buffer layer 34 may vary.

In one embodiment of the present application and as illustrated in FIG.3, a first surface of the silicon buffer layer 34 is in direct physicalcontact with a surface of the oxide layer 36 and a second a secondsurface of the silicon buffer layer 34 that is opposite the firstsurface directly contacts the n⁺ silicon layer 32.

In one embodiment of the present application, silicon handle substrate30 may have a thickness from 700 μm to 800 μm. Other thicknesses thatare lesser than, or greater than, the aforementioned thickness range mayalso be employed for the silicon handle substrate 30. In one embodimentof the present application, the n⁺ silicon layer 32 may have a thicknessfrom 1 micron to 10 microns. Other thicknesses that are lesser than, orgreater than, the aforementioned thickness range may also be employedfor the n⁺ silicon layer 32. In one embodiment of the presentapplication, the silicon buffer layer 34 may have a thickness from 100nm to 2000 nm. Other thicknesses that are lesser than, or greater than,the aforementioned thickness range may also be employed for the siliconbuffer layer 34. In one embodiment of the present application, the oxidelayer 36 may have a thickness from 10 nm to 10,000 nm. Other thicknessesthat are lesser than, or greater than, the aforementioned thicknessrange may also be employed for the oxide layer 36. In one embodiment ofthe present application, the p⁻ silicon layer 38 may have a thicknessfrom 5 nm to 10,000 nm. Other thicknesses that are lesser than, orgreater than, the aforementioned thickness range may also be employedfor the p⁻ silicon layer 38.

As mentioned above, presence of the silicon buffer layer 34 as shown inFIG. 3 between the oxide layer 36 and the n⁺ silicon layer 32 aids incontrolling the premature degradation of the oxide layer 36. In such anembodiment, the silicon buffer layer 34 minimizes injections ofelectrons from the n⁺ silicon layer 32 into the oxide layer 36 and thusprovides a more robust SOI substrate.

Referring now to FIGS. 4A, 4B and 5, there are illustrated a method thatcan be used in forming the SOI substrate shown in FIG. 3. Notably, FIG.4A illustrates a first wafer 50 that can be used in providing theexemplary semiconductor-on-insulator substrate illustrated in FIG. 3.First wafer 50 includes an oxide layer 36 located on a surface of aprecursor p⁻ silicon layer 38L. The precursor p⁻ silicon layer 38L isused in providing the p⁻ silicon layer 38 shown in FIG. 3. In oneembodiment of the present application, oxide layer 36 may be thermallygrown on the surface of the precursor p⁻ silicon layer 38L. In such anembodiment, an oxidation process can be used to provide the oxide layer36 on the precursor p⁻ silicon layer 38L. In another embodiment of thepresent application, the oxide layer 36 may be formed on the precursor psilicon layer 38L by utilizing a deposition process such as, forexample, chemical vapor deposition or chemical vapor deposition. Theprecursor p⁻ silicon layer 38L includes a layer releasing implant region(not shown) containing H⁺, He⁺ or a combination of H⁺ and He⁺ that isformed somewhere near an upper portion of the precursor p⁻ silicon layer38L. After bonding to the second wafer, an anneal may be used to cleavea portion of the precursor p⁻ silicon layer 38L using the layerreleasing implant region to initiate crack propagation and cleavage toprovide the p⁻ silicon layer 38 of the SOI substrate of the presentapplication.

FIG. 4B illustrates a second wafer 52 that can be used in providing theexemplary semiconductor-on-insulator substrate illustrated in FIG. 3.The second wafer 52 comprises, from bottom to top, a silicon handlesubstrate 30, an n⁺ silicon layer 32 and a silicon buffer layer 34. Asstated above, silicon buffer layer 34 is selected from undoped silicon,p-doped silicon or a multilayered stack of, in any order, undopedsilicon and p-doped silicon.

In one embodiment of the present application, the second wafer 52 may beformed by first providing the silicon handle substrate 30. The siliconhandle substrate 30 can be formed by introducing a p-type dopant (or ann-type dopant) into intrinsic silicon utilizing one of ion implantationor gas phase doping. A dopant activation anneal may follow the dopantintroduction step. Next, the n⁺ silicon layer 32 can be epitaxiallygrown on a surface of the silicon handle substrate 30, followed by theepitaxial growth of the silicon buffer layer 34 on a surface of the n⁺silicon layer 32.

The terms “epitaxial growth and/or deposition” and “epitaxially formedand/or grown” mean the growth of a semiconductor material on adeposition surface of a semiconductor material, in which thesemiconductor material being grown has the same crystallinecharacteristics as the semiconductor material of the deposition surface.In an epitaxial deposition process, the chemical reactants provided bythe source gases are controlled and the system parameters are set sothat the depositing atoms arrive at the deposition surface of asemiconductor material with sufficient energy to move around on thesurface and orient themselves to the crystal arrangement of the atoms ofthe deposition surface. Therefore, an epitaxial semiconductor materialthat is formed by an epitaxial deposition process has the samecrystalline characteristics as the deposition surface on which it isformed. For example, an epitaxial semiconductor material deposited ona{100} crystal surface will take on a {100} orientation.

Examples of various epitaxial growth processes that are suitable for usein the present application include, e.g., rapid thermal chemical vapordeposition (RTCVD), low-energy plasma deposition (LEPD), ultra-highvacuum chemical vapor deposition (UHVCVD), atmospheric pressure chemicalvapor deposition (APCVD), molecular beam epitaxy (MBE) or metal-organicCVD (MOCVD). The temperature for epitaxial deposition typically rangesfrom 250° C. to 1100° C. A number of well known silicon formingprecursor source gases may be used for the deposition of the n⁺ siliconlayer 32 and the silicon buffer layer 34. Carrier gases like hydrogen,nitrogen, helium and argon can be used.

In some embodiments, intrinsic silicon or an intrinsic silicon materialstack may be epitaxially grown, and thereafter the appropriate dopant ordopants can be introduced into the intrinsic silicon or the intrinsicsilicon material stack in order to provide the n⁺ silicon layer 32 andthe silicon buffer layer 34.

In yet another embodiment, the second wafer 52 can be formed by firstproviding a base silicon substrate. In one embodiment, the base siliconsubstrate may be undoped. In yet another embodiment, the base siliconsubstrate may be doped. Next, a p-type dopant (or an n-type dopant) canbe introduced within a lower portion of the base silicon substrate toprovide the silicon handle substrate 30. In some embodiments, and whenthe base substrate is p-type doped or n-type doped, this step may beomitted. Next, an n-type dopant can be introduced in a middle portion ofthe base silicon substrate to provide the n⁺ silicon layer 32. A dopantactivation anneal may follow the introduction of dopant within aparticular semiconductor material. In some embodiments, and when thebase substrate is n-type doped, this step may be omitted. In someembodiments, in which the base silicon substrate is undoped or p-typedopant, an upper portion of the base silicon substrate can provide thesilicon buffer layer 34. In yet another embodiment, p-type dopants canbe introduced into the upper portion of the base silicon substrate. Theintroduction of dopants can be performed by ion implantation or gasphase doping. A dopant activation anneal may follow the introduction ofdopant within a particular semiconductor material.

At this point of the present application, the oxide layer 36 of thefirst wafer 50, the silicon buffer layer 34 of the second wafer 52 orboth the oxide layer 36 of the first wafer 50 and the silicon bufferlayer 34 of the second wafer 52 can be subjected to a plasma activationprocess. The plasma activation process adds charge and/or dangle bondsto the oxide layer 36 and/or silicon buffer layer 34 which may aid insubsequent bonding of the first and second wafers. The plasma activationprocess can be performed in a plasma of O₂, N₂, an inert gas, or anymixture thereof. The plasma activation process can be performed attemperatures that are well known to those skilled in the art.

Referring now to FIG. 5, there is illustrated the structure that isformed after bringing the first wafer 50 shown in FIG. 4A into intimatecontact with the second wafer 52, bonding the first and second wafers(50, 52) together, and thereafter removing a portion of the precursor p⁻silicon layer 38L to provide an SOI in accordance with the presentapplication. Notably, and as is shown, a surface of the oxide layer 36(with or without plasma activation) of the first wafer 50 is broughtinto intimate contact with a surface of the silicon buffer layer 34(with or without plasma activation) of the second wafer 52. The bringingof the two wafers (50, 52) into intimate contact with each other may beperformed by hand or by mechanical means such as, for example, a robotarm. After bringing the two wafers (50, 52) into intimate contact witheach other, an external force or load may be applied to at least one ofthe two wafers. Next, bonding of the two contacting wafers is performed.The bonding is performed using any bonding temperature that forms asufficient bonding interface between the oxide layer 36 and the siliconbuffer layer 34. Bonding is typically performed in an inert ambientincluding, for example, He, Ar, or mixtures thereof. Bonding providesthe SOI substrate of the present application. Following bonding,cleaving of the precursor p⁻ silicon layer 38L is performed utilizingtechniques well known in the art to provide the p⁻ silicon layer 38 ofthe SOI substrate of the present application.

In some embodiments of the present application, an annealing can beperformed on the semiconductor-on-insulator substrate shown in FIG. 3 or5 to eliminate or reduce electron traps that are present in the oxidelayer 36. When such a step is conducted, the anneal can be performed ata temperature sufficient to cause electron trap reduction, whilemaintaining the integrity of the structure. The anneal may be performedin a variety of ambients including, but not limited to an inert ambientsuch as, for example He or Ar.

Although the annealing is mentioned for eliminating or reducing electrontraps that are present in the oxide layer 36 of the SOI substrate of thepresent application, the same annealing can be used to eliminate orreduce electron traps that are present in the oxide layer of any SOIsubstrate including the ones illustrated in FIGS. 1 and 2. As such, thepresent application also provides a method that includes providing asemiconductor-on-insulator (SOI) substrate having an oxide layer thatcontains a first quantity of electron traps. The method continues byannealing the SOI substrate as described above to provide the oxidelayer with a second quantity of electron traps, wherein the secondquantity of the electron traps is less than the first quantity of theelectron traps.

While the present application has been particularly shown and describedwith respect to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formsand details may be made without departing from the spirit and scope ofthe present application. It is therefore intended that the presentapplication not be limited to the exact forms and details described andillustrated, but fall within the scope of the appended claims.

What is claimed is:
 1. A semiconductor-on-insulator (SOI) substratecomprising from bottom to top, a silicon handle substrate; an n⁺ siliconlayer; a silicon buffer layer consisting of undoped silicon; an oxidelayer; and a p⁻ silicon layer.
 2. The SOI substrate of claim 1, whereina first surface of said silicon buffer layer is in direct physicalcontact with a surface of said oxide layer.
 3. The SOI substrate ofclaim 2, wherein a second surface of said silicon buffer layer that isopposite said first surface directly contacts said n⁺ semiconductorlayer.
 4. The SOI substrate of claim 1, wherein each of said siliconhandle substrate, said n⁺ silicon layer, and said silicon buffer layerhave an epitaxial relationship with each other.
 5. The SOI substrate ofclaim 1, wherein said silicon handle substrate is a p⁻ semiconductorsubstrate.
 6. A semiconductor-on-insulator (SOI) substrate comprisingfrom bottom to top, a silicon handle substrate; an n⁺ silicon layer; asilicon buffer layer consisting of p-doped silicon; an oxide layer; anda p⁻ silicon layer.
 7. The SOI substrate of claim 6 wherein said siliconbuffer layer has a p-dopant concentration of from 10¹⁴ atoms/cm³ to 10¹⁸atoms/cm³.
 8. The SOI substrate of claim 6, wherein a first surface ofsaid silicon buffer layer is in direct physical contact with a surfaceof said oxide layer.
 9. The SOI substrate of claim 8, wherein a secondsurface of said silicon buffer layer that is opposite said first surfacedirectly contacts said n⁺ semiconductor layer.
 10. The SOI substrate ofclaim 6, wherein each of said silicon handle substrate, said n⁺ siliconlayer, and said silicon buffer layer have an epitaxial relationship witheach other.
 11. The SOI substrate of claim 6, wherein said siliconhandle substrate is a p⁻ semiconductor substrate.
 12. Asemiconductor-on-insulator (SOI) substrate comprising from bottom totop, a silicon handle substrate; an n⁺ silicon layer; a silicon bufferlayer consisting of a multilayered stack of, in any order, undopedsilicon and p-doped silicon; an oxide layer; and a p⁻ silicon layer. 13.The SOI substrate of claim 12, wherein said undoped silicon of saidmultilayered stack is in direct physical contact with said n+ siliconlayer, and said p-doped silicon of said multilayered stack is in directphysical contact with said oxide layer.
 14. The SOI substrate of claim12, wherein said p-doped silicon of said multilayered stack is in directphysical contact with said n+ silicon layer, and said undoped silicon ofsaid multilayered stack is in direct physical contact with said oxidelayer.
 15. The SOI substrate of claim 12, wherein each of said siliconhandle substrate, said n⁺ silicon layer, and said silicon buffer layerhave an epitaxial relationship with each other.
 16. The SOI substrate ofclaim 12, wherein said silicon handle substrate is a p⁻ semiconductorsubstrate.